Ice Exists on Surface of Comet, But Most Lies Deeper

COLLEGE PARK, Md., July 4, 2009—Scientists for Deep Impact, the University of Maryland-led NASA mission that made history when it smashed into a comet this past July 4th, have added another first to their growing list: the first finding of water ice on the surface of a comet.

By analyzing data and images taken prior to impact, Deep Impact scientists have detected water ice in three small areas on the surface of comet Tempel 1. This is the first time ice has been detected on the nucleus, or solid body, of a comet. The findings are published today in the online version of the journal Science.

"These results show that there is ice on the surface, but not very much and definitely not enough to account for the water we see in the out-gassed material that is in the coma [the cloud of gas and dust that surrounds the comet]," said lead author Jessica Sunshine of Science Applications International Corporation.

"These new findings are significant because they show that our technique is effective in finding ice when it is on the surface and that we can therefore firmly conclude that most of the water vapor that escapes from comets is contained in ice particles found below the surface," said Deep Impact Principal Investigator Michael A'Hearn of the University of Maryland.

Where's the Ice?

Through observations of ice grains and water vapor in the coma of comets, scientists have long known that "dirty snowballs," as comets are sometimes described, must indeed contain substantial amounts of water ice. However, prior to Deep Impact they didn't have any knowledge about how such ice was distributed between the surface, subsurface and inner core of a comet's nucleus.

In the Science article, the authors say that prior to Deep Impact there existed few observations of nuclei not obscured by the coma. Among previous cometary missions, the most notable of such observations was the Deep Space-1 mission to comet Borrelly, which searched unsuccessfully for evidence of water ice and other volatiles on that comet's surface. Limited ground based observations of possibly bare cometary nuclei have also failed to find clear evidence of surface ice.

The fact that the Deep Impact team found water on the surface, but only in a few scattered places, all but eliminates the possibility that there is a lot of undetectable surface ice "just hiding in the surface darkness," explained Sunshine.

The surface ice that the team detected was not located where the impact later occurred. This means, Sunshine explained, that the water ice and water vapor the team already had found in analyses of material ejected by the July 4 impact must have come from ice located close to, but not on, the surface of the comet.